Centrifuge with a fluid line guide element having a curved bearing surface

ABSTRACT

A centrifuge having a rotating frame, which is rotatably mounted on a stationary frame, and a separation unit mounted on the rotating frame, rotating about a central axis of rotation in the same direction of rotation as the rotating frame but at twice the rotational speed. A line for supplying and/or removing a fluid is connected to the separation unit and is guided in a loop about the separation unit to a stationary tie-in point. To support the line, at least one guide element having a bearing surface for the line is provided. The bearing surface of the guide element is formed by a rotating planar curve, where the radius of the respective circle of curvature contacts the curve at a point and increases with increasing distance between the contact point and the central axis of rotation. With this contour, abrasion over the entire contact surface is largely uniform, which thereby lengthens the lifetime.

FIELD OF THE INVENTION

The present invention relates to a centrifuge, in particular acontinuous-flow centrifuge without face seals for centrifugingbiological fluids.

BACKGROUND OF THE INVENTION

Centrifuges are known in which the biological fluid is centrifuged incontinuous flow. The fluid is supplied to a rotating centrifuge chamberand removed from the chamber through a line. Because of the relativemovement of the centrifuge chamber and the stationary tie-in point ofthe line, however, guidance of the line has proven to be problematical.To prevent twisting of the line, rotating seals are used at the tie-inpoints of traditional continuous-flow centrifuges. Such continuous-flowcentrifuges allow a high rotational speed, but the rotational couplingsmay lead to leakage and damage to components contained in the fluid.

A blood centrifuge without face seals is disclosed in German Patent 3242 541 A1. With this centrifuge, which does not use face seals, the linepasses from a stationary tie-in point in a loop around the centrifugechamber. Therefore, the line is connected to a rotating frame whichrotates at half the rotational speed of the centrifuge chamber. Such acontinuous-flow centrifuge is known from German Patent 42 20 232 A1, forexample.

In the case of continuous-flow centrifuges that do not use face seals,the line is exposed to relatively great mechanical stresses thatincrease greatly with an increase in rotational speed. Under theinfluence of centrifugal forces, the line forms a loop protrudingoutward, so that high reversed bending loads occur at the stationarytie-in point and the connection to the separation chamber. The steepinlet and outlet angles at the tie-in points lead to additional frictionbetween the connection adaptors and the line, which in turn results inincreased abrasion. The reversed bending load and abrasion are thefactors which limit the lifetime of the line and the maximum rotationalspeed of the centrifuge.

Continuous-flow centrifuges that do not use face seals are known,whereby bearings are used to support the line. For example, EuropeanPatent 112 990 A1 describes a continuous-flow centrifuge in which thecentrifuge tubing is supported between the stationary tie-in point andthe connection of the separation chamber to two friction bearings. Thefriction bearings, consisting of inner and outer bearing shells, arecomponents of the centrifuge tubing. As disposable items, the frictionbearings are simple and economical to manufacture but they haverelatively high friction losses, particularly at high rotational speeds.

International Patent Application WO 95/17261 A1 describes acontinuous-flow centrifuge in which the centrifuge tubing is supportedwith a roller bearing mounted on the rotating frame. The roller bearing,consisting of the inner and outer bearing shells with the roller bodiesis part of the line. Although the roller bearing offers the advantage oflow bearing friction, it is complicated to manufacture and therefore theprice is relatively high. This is important inasmuch as the centrifugeline is a disposable item which is discarded after use. Acontinuous-flow centrifuge which does not use face seals and has abearing for supporting the fluid line is also known from German Patent198 03 535 A1.

U.S. Pat. No. 4,221,322 describes a continuous-flow centrifuge in whichthe fluid line is supported on a rotationally symmetrical bearingsurface having a section formed by a rotating arc. U.S. Pat. No.4,111,356 proposes a guide element for supporting the fluid line whichalso has a rotationally symmetrical bearing surface.

SUMMARY OF THE INVENTION

The object of the present invention is to create a centrifuge that canbe manufactured especially easily and inexpensively, whose line forsupplying and/or removing the fluid is adequately supported on the onehand while on the other hand being exposed to relatively low mechanicalstresses so that high rotational speeds are possible. In addition, it isespecially simple and inexpensive to manufacture the line.

The bearing surface of the guide element supporting the line of thecentrifuge according to the present invention is formed by a rotatingplanar curve, where the radius of the respective circle of curvature,which contacts the curve at a point, increases with an increase indistance between the point of contact and the central axis of rotationof the centrifuge. This yields the result that abrasion of the guideelement and the line is largely uniform over the entire bearing surface.It has been found by the present invention that the lifetime of the lineis improved by uniform abrasion.

The quotient of the respective radius of curvature R2 and the respectivedistance R1 between the contact point and the axis of rotation should beas constant as possible over the course of the curve describing thebearing surface of the guide element. The constant a=R2/R1 is greatlyinfluenced by the materials used and the geometric boundary conditions.In particular, the flexural rigidity of the line is important. It hasbeen found by the present invention that its lifetime can be increasedparticularly when 1≦a≦2.

In a preferred embodiment of the present invention, the guide element isa sleeve-shaped body which surrounds the line. Both the guide elementand the line are preferably made of abrasion-resistant materials whichcan slide easily on one another without the use of a lubricant.

In an especially preferred embodiment of the present invention, thebearing surface of the guide element is connected to a cylindrical guidesection whose diameter is preferably such that the line is guidedloosely in it. The cylindrical guide section is used to guide a straightsection of the line, while the bearing surface serves to support a bentline section downstream from the former.

To support the line, the centrifuge preferably has two guide elements.The first guide element is provided on the side of the separation unitwhich faces away from the stationary tie-in point. With this guideelement, the line extending away from the separation unit is guided in aloop around the separation unit to the stationary tie-in point,whereupon a first section of the line is supported on the bearingsurface of the guide element.

The second guide element which is provided at the stationary tie-inpoint is supported with its second bearing surface on a second sectionof the line which is guided about the separation unit.

With the bearing surfaces according to the present invention on the twoguide elements, the line is supported so that it is exposed torelatively low mechanical stresses so that high rotational speeds arepossible even after lengthy standing times. In principle, however, onlythe one guide element or the other may have the bearing surfaceaccording to the present invention.

Depending on the shape of the loop in which the line is to be guided,other guide elements may also be provided. Each guide element may alsohave two of the bearing surfaces according to the present invention toprovide support for the line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a greatly simplified schematic diagram of an embodiment ofa continuous-flow centrifuge without face seals for centrifugingbiological fluids, in particular blood;

FIG. 2 shows a schematic diagram of one of the two guide elements of thecentrifuge from FIG. 1 in an enlarged drawing together with a section ofthe centrifuge line;

FIG. 3 shows the contour of the bearing surface of the guide elementfrom FIG. 2; and

FIG. 4 shows additional embodiments of different contours of the bearingsurface of the guide element from FIG. 2.

DETAILED DESCRIPTION

A representative embodiment of the present invention is described ingreater detail below with reference to the drawings.

FIG. 1 shows a schematic diagram of a continuous-flow centrifuge thatdoes not use face seals for centrifuging a biological fluid, inparticular blood, which corresponds in design and function to thecentrifuge described in German Patent 32 42 541 A1. The continuous-flowcentrifuge has a rotating frame 1 with a lower supporting plate 1 a andan upper supporting plate 1 b plus two side parts 1 c, 1 d. Rotatingframe 1 is rotatably mounted on a stationary frame 2 to rotate about avertical axis 3, and it is driven by a drive unit (not shown in FIG. 1)at a rotational speed n₁. A separation unit 4 in the form of acylindrical chamber is mounted on the upper supporting plate 1 b of therotating frame 1 so it can rotate about the axis of rotation of rotatingframe 1. Separation unit 4 is driven by a drive unit (not shown) in thesame direction of rotation as the rotating frame 1 but at twice therotational speed n₂=2n₁. Separation unit 4 may also be mounted on thelower side of supporting plate 1 b.

The central axis of rotation, about which the separation unit 4 as wellas the rotating frame 1 rotate, is labeled with reference notation A inFIG. 1.

A flexible line 6, which combines one or more tubes for supplying andremoving the blood or blood constituents into and out of separation unit4, leads from a stationary tie-in point 5 of the centrifuge frame andaround separation unit 4 and is connected to its lower side. Twisting ofthe line is prevented by the fact that line 6 passes around theseparation unit at half the rotational speed in comparison withseparation unit 4.

Line 6 is part of a disposable set which includes, in addition toseparation unit 4, bags for collecting the blood components separated bycentrifugation. The disposable set is inserted into the centrifuge anddiscarded after use. Such tubing arrangements are well known in the art,and as such would be familiar to and readily understood by one ofordinary skill in the art. For example, a disposable set includingmultiple collecting bags is described in German Patents 28 45 364 A1 and28 45 399 A1, which are hereby incorporated by reference in theirentireties.

To reduce the mechanical stress on line 6 due to centrifugal forces, itis supported at the stationary tie-in point 5 as well as at the side ofthe separation unit 4, which faces away from the stationary tie-in point5. Therefore, a first guide element 7 is provided on the lower side ofthe separation unit, supporting a first section 6 a of line 6. A secondguide element 8 is provided at the stationary tie-in point 5 to supporta second line section 6 b. The two guide elements 7, 8 have an identicalstructure. The line may be attached to a side arm 9 at the uppersupporting plate 1 b of the frame with another guide element (not shown)or bearing.

FIG. 2 shows a schematic diagram of one of the two guide elements 7, 8of the centrifuge, which is described with reference to FIG. 1 in asectional diagram.

Guide element 7, 8 is a sleeve-shaped body made of ABS plastic inparticular. It has a rotationally symmetrical bearing face 10 forsupporting a curved section 11 of line 6, to which is connected acylindrical guide face 12 for guiding a straight section 13 of line 6.Cylindrical guide face 12 has a diameter slightly greater than theoutside diameter of line 6, so that the line is loosely guided in theguide element.

Rotationally symmetrical bearing face 10 of guide element 7, 8 is formedby a planar curve 15 rotating about longitudinal axis 14 of the guideelement, the course of which is just indicated in FIG. 2. Longitudinalaxis 14 of the guide element lies on the axis of rotation A of thecentrifuge.

Without intending to be limited to the theory of how the presentinvention works, what follows is a description of the theoreticalbackground to explain abrasion of line 6 and guide elements 7, 8 as wellas the exact contour of the bearing surface for minimizing abrasion andprolonging the lifetime of both the line and the guide element.

The abrasion of two surfaces rubbing against one another can bedescribed in first approximation by the following equation:

A˜V*P

Abrasion A is proportional to the relative velocity V of the twosurfaces rubbing against one another and their surface pressure P.

In the case of the tubing connection, the relative velocity is notconstant but instead is proportional to the radial distance R1 from thecentral axis of rotation A of the centrifuge.

V˜R1

The surface pressure depends on forces which are caused by bending ofthe line and by tensile force due to the centrifugation.

P=P 1+P 2

P1 increases in proportion to the smaller bending radius R2 according tothe laws of the bending beam, and P2 also increases in proportion to thedecrease in the bending radius R2 according to the laws of cablelooping. The following thus holds for the total surface pressure P:

P˜1/R2

This yields the following equation for abrasion:

A˜R1/R2

The contour of bearing surface 10 of guide element 7, 8 should be suchthat abrasion remains as unchanged as possible over the entire bearingsurface. It follows from this that the quotient of the two radii R1 andR2 is as constant as possible over the course of curve 15 whichdescribes the bearing surface.

R 2/R 1=a

FIG. 3 shows the exact curve describing the bearing surface. Let usassume that the central axis of rotation A of the centrifuge passesthrough the point of origin of the Cartesian coordinate system (y axis).

The course of the curve is described by the following equations:

x _((i+1)) =R _(1(i+1)) =R _(1i) +R _(li) *a*(cosα_(i)−cos(α_(i)+Δα))  (1)

 Y _((i+1)) =Y _(l) +R _(li) *a*(−sin α₁+sin(α₁+Δα))  (2)

Δα=Δα₁=const.  (3)

α₁ =i*Δα  (4)

The radius R2 of the respective circle of curvature, which contacts thecurve at a point, increases with an increase in distance R1 between thepoint of contact and the central axis of rotation.

For example, the circle of curvature contacting the curve at point P_(i)has radius of curvature R_(2i). The distance between contact point P_(i)and central axis of rotation A (y axis) is R_(1i)=x_(i). The circle ofcurvature contacted by the curve at point P_(i+1) has the radiusR_(2i+1). The distance between the point of contact P_(i+1) and the axisA of rotation (y axis) is R_(1i+1)=x_(i+1). As shown in FIG. 3, thepoint of contact P_(1+l) is a greater distance from the y axis than isthe point of contact P_(i). Consequently, the radius of curvature atpoint P_(i+1) must be greater than the radius of curvature at pointP_(i), which is shown in FIG. 3. The middle points of the circles ofcurvature are labeled as M_(i,i+1) in FIG. 3.

According to the present invention, it has been found in practice thatlong lifetimes can be achieved particularly when 1≦a<2. FIG. 4 shows thecontour of the bearing surface for a=1, a=1.6 and a=2. Preferably, thecontour should run between the limits of a=1 and a=2.

At a constant a=1.6, the abrasion is distributed largely uniformly overthe entire surface of contact. In the case of a centrifuge rotating at arotational speed of 4,000 L/min, the lifetime could be increased toapproximately nine hours with such a contour. The curve describing thebearing surface may also be approximated with an ellipse.

What is claimed is:
 1. A centrifuge comprising: a stationary frame; arotating frame which is rotatably mounted on the stationary frame; aseparation unit mounted on the rotating frame, wherein the separationunit rotates about a central axis of rotation in the same direction ofrotation as the rotating frame but at twice the rotational speed; a linefor at least one of supplying and removing at least one fluid, whereinthe line is connected to the separation unit and guided in a loop aboutthe separation unit to a stationary tie-in point; and at least one guideelement having a bearing surface, wherein the line is supported on thebearing surface and the bearing surface is formed by a rotating planarcurve; wherein at a contact point, P, on the rotating planar curve, arespective circle of curvature contacts the rotating planar curve, andwherein the radius of the respective circle of curvature increases withan increase in the distance between the contact point, P, and thecentral axis of rotation.
 2. The centrifuge according to claim 1,wherein the quotients of the radius of the respective circle ofcurvature divided by the distance between the contact point, P, and thecentral axis of rotation is equal to a constant, a.
 3. The centrifugeaccording to claim 2, wherein the value of the constant, a, satisfiesthe following equation: 1≦a≦2.
 4. The centrifuge according to claim 1,wherein the at least one guide element comprises a sleeve-shaped body.5. The centrifuge according to claim 1, wherein the bearing surface isconnected to a cylindrical guide surface.
 6. The centrifuge according toclaim 5, wherein the diameter of the cylindrical guide surface isslightly greater than the outside diameter of the line, such that theline is loosely guided in the at least one guide element.
 7. Thecentrifuge according to claim 1, wherein the at least one guide elementcomprises a first guide element having a bearing surface, wherein thefirst guide element is provided on the side of the separation unit thatfaces away from the stationary tie-in point, and wherein a first sectionof the line is supported on the bearing surface of the first guideelement.
 8. The centrifuge according to claim 7, wherein the at leastone guide element further comprises a second guide element having abearing surface, wherein the second guide element is provided at thestationary tie-in point, and wherein a second section of the line issupported on the bearing surface of the second guide element.
 9. Thecentrifuge according to claim 1, wherein the at least one guide elementcomprises a second guide element having a bearing surface, wherein thesecond guide element is provided at the stationary tie-in point, andwherein a second section of the line is supported on the bearing surfaceof the second guide element.